Method for optimizing the burnout of exhaust gases of an incinerator

ABSTRACT

A method for optimizing the burnout of exhaust gases of an incinerator includes introducing a solid material via an inlet into a combustion chamber defining a primary combustion space, combusting the solid material in the primary combustion space, in the form of a combustion bed conveyed over a combustion grate, with admission of primary air, discharging the combusted solid material from the primary combustion space via an outlet arranged opposite the inlet in the conveying direction, combusting the primary combustion gases released during the combustion of the solid material, with admission of secondary air, in a secondary combustion chamber defining a secondary combustion space and arranged downstream of the combustion chamber in the flow direction of the combustion gases, and homogenizing the exhaust gases containing the primary combustion gases in a mixing zone by means of a fluid introduced via a nozzle before entry into the secondary combustion space.

BACKGROUND

The present invention relates to a method for optimizing the burnout ofexhaust gases of an incinerator according to the preamble of claim 1 andalso to a combustion chamber for carrying out the method and to a wasteincinerator comprising such a combustion chamber.

Incinerators for combusting solid fuels, such as municipal waste,substitute fuels, biomass and other materials, are best known to aperson skilled in the art. Such facilities comprise a combustionchamber, in which the solid material is combusted with admission ofprimary air, which is referred to as primary combustion. Here, the solidmaterial passes through different sub-processes from the inlet into thecombustion chamber to the outlet, said sub-processes being dividedroughly into drying, ignition, combustion and ash burnout.

In each of these sub-processes, exhaust gases of different compositionare generated. Whereas in the drying phase the primary air merelyabsorbs moisture from the solid material to be combusted, pyrolyticdecomposition products are found in the ignition phase. In contrast tothe drying phase, the oxygen fed in the ignition phase is oftenconverted fully, such that the exhaust gas flow generated in this phaseincludes only very little oxygen or even no oxygen. Exhaust gases withtypical compositions of CO, CO₂, O₂, H₂O and N₂ are produced in thecombustion phase, whereas practically unconsumed air is ultimatelypresent above the ash burnout.

These different exhaust gas flows, after the primary combustion,generally reach a secondary combustion chamber arranged downstream inthe flow direction, where they are burned out with admission ofsecondary air, which is referred to as secondary combustion.

A method comprising a combustion of the solid material and a secondarycombustion of the incompletely burned exhaust gas constituents is knownfor example from WO 2007/09510, which has the objective of breaking downthe primary nitrogen compounds NH₃ and HCN in order to minimize theformation of nitrogen oxides (NO_(x)) in the secondary combustionchamber.

EP-A-1077077 concerns a method similar to that in WO 2007/090510,wherein the SNCR method is used for NO_(x) removal from flue gases, inwhich no catalyst is used, but instead a reducing agent is injected intothe flue gases. Such SNCR methods operate at temperatures from 850 to1000° C. and require elaborate regulation.

The reduction of nitrogen oxides is additionally addressed in WO99/58902. In accordance with the method described therein, the gasesexiting from the combustion chamber are homogenized in a mixing stagewith addition of a medium free from oxygen or low in oxygen, after whichthe homogenized exhaust gas flow passes through a steady-state zone, inwhich the nitrogen oxides already formed are to be reduced. Depending onthe operating conditions, it may be that the quantity of accumulatingpyrolysis gas is of such a size that the quantity of locally availablesecondary air is not sufficient for complete burnout. As a result,unburned gases escape from the secondary combustion chamber andprecipitate for example in CO peaks in the flue.

As a result of the various combustion zones, a temperature imbalance isalso produced in addition to the differences in the composition of theexhaust gas flows. A much higher temperature is thus present in theignition and the combustion zone compared for example to the ash burnoutzone. This imbalance is intensified further in the secondary combustionchamber, since the exhaust gases generated in the ignition andcombustion zone have a higher proportion of combustible primarycombustion gases compared to the exhaust gas generated in the ashburnout zone, and the combustion of these combustible gases increasesthe temperature additionally.

Particularly in the inlet-side region, the peripheral wall surroundingthe combustion chamber or the secondary combustion chamber can bedamaged by the prevailing high temperatures. In addition, caking orcoking may occur in this region due to the high temperatures and has tobe removed in complex maintenance procedures.

The methods described in EP-A-1081434, EP-A-1382906 and U.S. Pat. No.5,313,895 for example attempt to overcome the problem of reducing thequantity of unburned substances and in particular CO. For example, inaccordance with U.S. Pat. No. 5,313,895, a mixing fluid is introducedwhich causes the gases exiting from the combustion chamber to be swirledin an eddy current. In addition, to introduce the fluid, a specialnozzle arrangement is described for example in EP-A-1081434, as a resultof which a rotating flow is generated in the flow channel in aninjection plane arranged in the region of the flame cover. However, themethod described in particular in U.S. Pat. No. 5,313,895 only takesinto account unsatisfactorily the problem of the temperature imbalancepresent in the combustion chamber. In accordance with said document, thetemperature in the inlet-side region in the combustion chamber is to bereduced by means of injection of water droplets or water vapor. This isdisadvantageous however in view of the energy recovery balance. Theobjective of the present invention is therefore to provide a method foroptimizing the burnout of exhaust gases of an incinerator, said methodon the one hand ensuring high operational reliability and on the otherhand allowing a high energy recovery from the combustion process.

SUMMARY

A method relating to an embodiment of the invention consequentlyincludes the steps of introducing the solid material to be combusted viaan inlet into a combustion chamber defining a primary combustion space,combusting the solid material in the primary combustion space, in theform of a combustion bed conveyed over a combustion grate, withadmission of primary air, and discharging the combusted solid materialfrom the primary combustion space via an outlet arranged opposite theinlet in the conveying direction.

The primary combustion gases released during the combustion of the solidmaterial are combusted, with admission of secondary air, in a secondarycombustion chamber defining a secondary combustion space and arrangeddownstream of the combustion chamber, that is to say generally above thecombustion chamber, in the flow direction of the combustion gases.

Before entry into the secondary combustion space, that is to sayupstream in the flow direction and therefore generally below thecombustion space, the exhaust gases containing the primary combustiongases are homogenized in a mixing zone. This occurs by means of a fluidintroduced via a nozzle.

In this context “a” (nozzle) is to be understood as the indefinitearticle; the term includes both a single nozzle and also a plurality ofnozzles.

In this context, the term homogenization is understood to mean that theexhaust gases or the individual exhaust gas flows of differentcomposition are mixed in such a way that a gas mixture that is ashomogeneous as possible is obtained. In accordance with the invention,the mixing zone then adjoins the combustion bed at least approximatelydirectly in the flow direction of the exhaust gases. It is therefore inother words generally arranged at least approximately directly above thecombustion bed. This allows very hot exhaust gas flows, for example asmay be produced in the ignition or combustion zone, to mix practicallydirectly above the combustion bed with the cooler exhaust gas flows fromthe drying and ash burnout zones and therefore to compensate for or toreduce temperature peaks in good time. At the same time, the methodprevents the energy recovery balance from being impaired, for example aswould be the case with cooling by means of a cooling medium.

In addition, a gas mixture is obtained as a result of the homogenizationof the exhaust gas flows generated in the individual combustion zonesand is optimally preconditioned for the secondary combustion in thesecondary combustion space. As a result, it is possible to ensureoptimal burnout of the exhaust gases, even with low (secondary) airexcess; the emission of harmful substances, such as CO or unburnedhydrocarbons, can thus be kept very low, even with small quantities ofadmitted secondary air.

It has also been found that the mixture of the reducednitrogen-containing combustion gases (nitrogen oxide precursorsubstances) generated in the combustion zone with the oxygen presentabove the drying or burnout zone does not result in an increase innitrogen oxides. This can be explained by the fact that, as the exhaustgas flow from the combustion zone is mixed with the oxygen-rich exhaustgas flows accumulating in the drying and burnout zones, the temperatureof said gas flows is simultaneously reduced, which suppresses theformation of thermal NO_(N).

As mentioned above, the fluid is introduced via one or more nozzles.

The exit speed of the fluid from the nozzle is approximately 40 toapproximately 120 m/s, wherein, within the meaning of the presentinvention, the nozzle is oriented at an angle from −10° to +10° relativeto the inclination of the combustion grate.

In addition to the above-defined nozzles, further nozzles can beprovided which are not aligned relative to the inclination of thecombustion grate at the above-defined angle.

In this context, the inclination of the grate is understood to mean thetotal inclination of the grate (and not the orientation of anyindividual grate steps present).

Due to the orientation of the nozzle, it is ensured that excessiveswirling of solid materials by the grate is avoided, even with thearrangement of the mixing zone directly above the combustion bed.

The injection speed of the fluid from approximately 40 to approximately120 m/s also helps to avoid a swirling of solid materials.

The discovered combination of nozzle arrangement and injection speedtherefore on the whole enables the mixing zone to adjoin the combustionbed at least approximately directly in the flow direction of the exhaustgases without resulting in an excessive undesired swirling of the solidmaterials by the combustion grate.

The fact that good homogenization can be obtained already with theinjection speed from approximately 40 to approximately 120 m/s is allthe more surprising since significantly higher values are taught in theprior art. For example, an exit speed of at least 1 MACH is disclosedfor example in EP-A-1508745. A MACH number of 1 is synonymous with thespeed of sound, which for air at 20° C. is generally specified at 343m/s, and adopts even higher values at higher temperatures as are to befound in furnaces.

In accordance with a preferred embodiment, the distance between themixing zone and the combustion bed is at most 1.5 meters, preferably atmost 0.8 meters. This distance therefore denotes the maximum distancebetween the upper limit of the combustion bed and the start of themixing zone as considered in the flow direction of the exhaust gases.Said maximum distance, in view of the conventional dimensions of anincinerator, still falls within the expression “approximately above thecombustion bed”. Since the upper limit of the combustion bed istypically arranged approximately 0.3 to 1 meter above the surface of thecombustion grate, the mixing zone is distanced appropriately from thecombustion grate.

In accordance with a further preferred embodiment, the mixing zoneextends at most up to a distance of 2 meters measured from thecombustion bed. As considered in the flow direction of the exhaustgases, the mixing zone in accordance with this embodiment thus endsafter 2 meters at most and therefore still at a sufficient distancebefore the secondary air injection. In the case of the mixing zoneadjoining the combustion bed at least approximately directly inaccordance with the invention, the mentioned upper limit is sufficientto obtain the desired homogenization of the exhaust gases.

A particularly good homogenization is achieved if, in accordance with apreferred embodiment, the exit speed of the fluid from the nozzle isapproximately 90 m/s.

Here, the exit speed refers to the speed that the fluid has as it exitsfrom the nozzle opening. The nozzles used as standard generally have acircular nozzle cross section from 60 mm to 200 mm. It is conceivablefor the nozzle cross section to taper continuously in the direction ofthe nozzle mouth, such that the diameter of the exit opening of thenozzle is 60 mm to 90 mm.

In order to minimize a swirling of the solid materials caused by theintroduction of the fluid, the respective nozzle is preferably orientedat an angle of −10° to +5°, preferably from −5° to +5°, relative to theinclination of the combustion grate. In accordance with a furtherpreferred embodiment, the respective nozzle is aligned at an angle from−10° to 0° relative to the inclination of the combustion grate.

In accordance with a further preferred embodiment, the fluid can be aflue gas returned from a subsequent zone downstream of the secondarycombustion space. In waste incinerators of conventional design, thereturn is preferably implemented here from a zone between the steamgenerator and the flue. The quantity of introduced flue gas is generallyapproximately 5 to 35% of the admitted quantity of primary air,preferably approximately 20%. Alternatively or additionally to the fluegas, any other conceivable fluid can be used, in particular air, aninert gas, such as nitrogen, water vapor or mixtures thereof.

Since the highest temperatures are generally present in the inlet-sideregion of the combustion chamber, the fluid is injected in accordancewith a preferred embodiment via a nozzle or row of nozzles arranged inthis region. A very pronounced temperature imbalance and thereforedamage or contamination of the peripheral wall surrounding thecombustion space can thus be effectively prevented.

In particular if a returned flue gas is then used as fluid, therespective nozzle preferably has an outer pipe and an inner pipe runningin the axial direction of the outer pipe and surrounded thereby, whereinthe inner pipe is intended to carry the flue gas and the outer pipe isintended to carry air. The inner diameter of the inner pipe ispreferably approximately 70 mm here, whereas the inner diameter of theouter pipe, that is to say the outer diameter of the annular gap presentbetween the inner pipe and outer pipe, is approximately 110 mm.

In this embodiment, the airflow is used as a shield, which protects thenozzle against the attachment of impurities entrained in the flue gas.Particularly at the temperatures present in the inlet-side region, suchattachments could easily lead to caking, which in the extreme case couldlead to failure of the nozzle; this is effectively prevented inaccordance with the presented embodiment.

It has proven to be advantageous if at least 1 nozzle is provided permeter of the combustion chamber width. The fluid is preferablyintroduced via at least two nozzles, preferably at least six nozzles.This ensures homogenization that is as complete as possible with arelatively small quantity of injected fluid.

Another embodiment relates to a combustion chamber for carrying out amethod. This chamber includes a peripheral wall enclosing a primarycombustion space, an inlet for introducing the solid material to becombusted into the primary combustion space, a combustion grate forcombusting the solid material, an outlet arranged opposite the inlet inthe conveying direction of the solid material for discharge of thecombusted solid material from the primary combustion space, and a nozzlefor homogenizing the exhaust gases containing the primary combustiongases released during the combustion process. Here, the nozzle isarranged in accordance with the invention in a range of at most 3meters, preferably 0.5 meters to 3 meters, most preferably 0.5 to 2meters, above the combustion grate.

The nozzle is generally arranged in the peripheral wall of thecombustion chamber, preferably in the region of the inlet or the outlet.

In order to avoid the homogenization process being accompanied by aswirling of the solid material present in the combustion bed, the nozzleis oriented in accordance with the invention at an angle from −10° to+10°, preferably from −10° to +5°, more preferably from −5° to +5°,relative to the inclination of the combustion grate. In accordance witha further preferred embodiment, the respective nozzle is oriented at anangle from −10° to 0° relative to the inclination of the combustiongrate.

Besides the described method and the described combustion chamber, anembodiment related to the present invention includes a waste incineratorwith a combustion chamber as described.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be illustrated with reference to the accompanyingfigures, in which

FIG. 1 shows a schematic illustration of a combustion chamber and asecondary combustion chamber (illustrated in part) for carrying out themethod according to the present invention; and

FIG. 2 shows a graph of the measured O2 concentration (in vol %) and COconcentration (in mg/m3 in standard ambient conditions) over time in anexhaust gas flow generated in the combustion zone, wherein the nozzlesare switched on and off at individual intervals.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As shown in FIG. 1, the solid material 2 to be combusted is poured intoa feed hopper 4 and is introduced from here into the combustion chamber8 generally by means of a dosing tappet via an inlet 6. The combustionchamber 8 comprises a peripheral wall 10, which surrounds an upwardlytapering primary combustion space 12.

The solid material 2 is conveyed in the form of a combustion bed 14above a (feed) combustion grate 16, through which primary air flows, andis combusted during the process. Here, a drying zone, an ignition zone,a combustion zone and an ash burnout zone are provided in succession inthe conveying direction F before the combusted solid material isdischarged via an outlet 18 arranged opposite the inlet 6 and is thenfed via a slag remover of a slag conveying apparatus. The primary air inthe shown embodiment is distributed via individual underblast chambers20 a, 20 b, 20 c, 20 d, which are fed via separate primary air lines 22a, 22 b, 22 c, 22 d.

Nozzles 24 a, 24 b, 24 c, indicated in FIG. 1 by means of arrows, arearranged in the peripheral wall 10 of the combustion chamber and areused to introduce a fluid into the combustion chamber 8.

Here, the nozzles are designed in such a way that the exit speed of thefluid from the nozzles is 40 to 120 m/s.

In the shown embodiment, a nozzle 24 a is arranged in the inlet-sideregion 8′ of the combustion chamber 8, specifically in a part 10′ of theperipheral wall 10, said part facing the inlet and running upwardly atan incline. Two nozzles 24 b, 24 c are arranged in the outlet-sideregion 8″, wherein one nozzle 24 b is arranged in the part 10″ runningupwardly at an incline and one nozzle is arranged in the part 10″ of theperipheral wall defining the end face 25 and running perpendicularly.Any other number and arrangement of nozzles suitable for the purposes ofthe present invention is also conceivable however.

By means of the nozzles 24 a, 24 b, 24 c, the exhaust gases, whichcontain the combustion gases released during the combustion process, arehomogenized in a mixing zone 26 adjoining the combustion bed 14 at leastapproximately directly in the flow direction of said gases. Thishomogenization is indicated in the figure by means of dashed arrows,wherein A schematically denotes the region with relatively hightemperature and relatively high concentration of primary combustiongases, and B denotes the region of lower temperature and lowerconcentration of primary combustion gases. After homogenization, that isto say in the figure above the regions denoted by A and B, the exhaustgases are present in the form of a homogeneous gas mixture.

This flows into a secondary combustion chamber 28 subsequent to thecombustion chamber 8 and defining a secondary combustion space 27, theexhaust gases being combusted in said secondary combustion chamber withadmission of secondary air. To this end, further nozzles 32 a, 32 b forintroduction of the secondary air are provided in the peripheral wall 30of the secondary combustion chamber 28.

As illustrated in FIG. 2, the introduction of the fluid with actuatednozzle in the ON position causes the O₂ concentration measured locallyin the exhaust gas flow generated in the combustion zone (shown in thicksolid lines) to correspond approximately to the global O₂ concentration,that is to say the total O₂ concentration present in the exhaust gasgenerated in the combustion chamber (shown in thin dashed lines). Bycontrast, with unactuated nozzle in the OFF position, the locallymeasured O₂ concentration is much lower than the globally measured O₂concentration.

With regard to CO concentration, a relatively low, approximatelyconstant value is obtained with actuated nozzle, whereas relatively highand significantly diverging values are obtained with unactuated nozzle,which further illustrates the homogenization of the exhaust gases by theintroduction of the fluid.

1. A method for optimizing the burnout of exhaust gases of anincinerator, said method comprising: introducing a solid material to becombusted via an inlet into a combustion chamber defining a primarycombustion space; combusting the solid material in the primarycombustion space, in the form of a combustion bed conveyed over acombustion grate, arc combusted with admission of primary air;discharging the combusted solid material from the primary combustionspace via an outlet arranged opposite the inlet in a conveyingdirection; combusting primary combustion gases released during thecombustion of the solid material, with admission of secondary air, in asecondary combustion chamber defining a secondary combustion space andarranged downstream of the combustion chamber in the flow direction ofthe combustion gases; and homogenizing the exhaust gases containing theprimary combustion gases in a mixing zone by means of a fluid introducedvia a nozzle before entry into the secondary combustion space, whereinthe mixing zone adjoins the combustion bed at least approximatelydirectly in the flow direction of the exhaust gases, the exit speed ofthe fluid from the nozzle is about 40 to about 120 m/s, and wherein thenozzle is oriented at an angle from about −10° to about +10° relative tothe inclination of the combustion grate.
 2. The method as claimed inclaim 1, wherein the distance between the mixing zone and the combustionbed is at most 1.5 meters.
 3. The method as claimed in claim 1, whereinthe mixing zone extends at most up to a distance of 2 meters measuredfrom the combustion bed.
 4. The method as claimed in claim 1, whereinthe exit speed of the fluid from the nozzle is about 90 m/s.
 5. Themethod as claimed in claim 1, wherein the nozzle is oriented at an anglefrom about −5° to about +5° relative to the inclination of thecombustion grate.
 6. The method as claimed in claim 1, wherein the fluidcomprises a flue gas returned from a subsequent zone downstream of thecombustion chamber.
 7. The method as claimed in claim 6, wherein thequantity of introduced flue gas is about 5% to about 35%, of theadmitted quantity of primary air.
 8. The method as claimed in claim 1,wherein the fluid is injected via a nozzle arranged in an inlet-sideregion of the combustion chamber.
 9. The method as claimed in claim 1,wherein the nozzle has an outer pipe and an inner pipe running in theaxial direction of the outer pipe and surrounded thereby, the inner pipebeing intended to carry the flue gas and the outer pipe being intendedto carry air.
 10. The method as claimed in claim 1, wherein the fluid isintroduced via at least two nozzles.
 11. A combustion chamber forcarrying out the method as claimed in claim 1, said combustion chambercomprising: a peripheral wall enclosing the primary combustion space;the inlet for introducing the solid material to be combusted into theprimary combustion space; a combustion grate for combusting the solidmaterial, the outlet arranged opposite the inlet in the conveyingdirection of the solid material for discharge of the combusted solidmaterial from the primary combustion space; and the nozzle forhomogenizing the exhaust gases containing the primary combustion gasesreleased during the combustion process, wherein the nozzle is arrangedin a range of at most 3 meters above the combustion grate, and whereinthe nozzle is oriented at an angle of about −10° to about +10° relativeto the inclination of the combustion grate.
 12. The combustion chamberas claimed in claim 11, wherein the nozzle is arranged in the peripheralwall of the combustion chamber.
 13. The combustion chamber as claimed inclaim 11, wherein the nozzle is arranged in the region of the inlet. 14.The combustion chamber as claimed in claim 11, wherein the nozzle isoriented at an angle from about −5° to about +5° relative to theinclination of the combustion grate.
 15. A waste incinerator, comprisinga combustion chamber as claimed in claim
 11. 16. The method as claimedin claim 2, wherein the distance between the mixing zone and thecombustion bed is at most 0.8 meters.
 17. The method as claimed in claim7, wherein the quantity of introduced flue gas is approximately 20% ofthe admitted quantity of primary air.
 18. The method as claimed in claim10, wherein the fluid is introduced via at least six nozzles.
 19. Thecombustion chamber as claimed in claim 11, wherein the nozzle isarranged in a range of about 0.5 meters to about 3 meters above thecombustion grate.
 20. The combustion chamber as claimed in claim 19,wherein the nozzle is arranged in a range of about 0.5 meters to about 2meters above the combustion grate.